The file system may be thought as an upside down tree. A node of the tree may be a special file, a directory files or an ordinary file. Under a directory node, a sub-tree having the same structure may be built.

A picture of the file system would start with the main file directory called the root and represented by a single /. The various system files, utilities and application programs reside in levels stemming from the root. With sufficient privilege, a user or program can “walk” around the tree to find any file in the system by traversing the proper branches as shown in the following figure.

Each registered user has a directory assigned by the system administrator. This is called “home directory” in which private files are stored.

The directory “/usr/local” and “/usr/apps” store all applications and utilities. The directory “/tmp” is used to stored the temporary working files of programs, and all files(and directories) under /tmp will be removed when the system restart.